Concanavalin A (Con A) activates human basophils to release histamine. This reaction has great similarities to that of antigen-or anti-IgE-induced release. It is characterized by a two stage reaction: the initial stage is the binding of Con A to the cell and the second stage is a secretory process of the cell. Cells desensitized by incubation with excess amounts of Con A or by prolonged incubation in the absence of cations cannot be activated to release with antigen or anti-IgE. When the native (tetravalent) Con A molecule is modified by succinic anhydride to become divalent it is much less active in releasing histamine.

There is wide variation in the response of cells from different individuals to Con A. However, a close correlation exists between the histamine release induced by Con A and anti-IgE. Therefore, in general, cells from allergic donors release more histamine with Con A than those from non-allergic individuals.

Evidence is presented to demonstrate that Con A acts by binding to carbohydrate groups on the IgE molecule located on the basophil membrane. On a molar basis, IgE is 4000-fold more active than α-methyl-D-mannoside in inhibiting the Con A-induced histamine release. Succinyl Con A blocks the ability of Con A and anti-IgE equally well and to a lesser extent the ability of antigen to activate cells to release histamine.

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